Low-pressure CVD apparatus and method of manufacturing a thin film

ABSTRACT

An LPCVD apparatus comprising: a container for accommodating an organometallic compound which serves as a raw material; a heating means for heating the container and vaporizing the organometallic compound to obtain a raw material gas; a reactor for accommodating a substrate on which a thin film being precipitated; an exhaust pump for maintaining a low pressure atmosphere within the reactor; and a trap provided on the upstream side of the exhaust pump and cooling used raw material gas supplied from the reactor. In the reactor, the trap is provided with honeycomb-structure cylindrical fillers in a flowing passage through which the used raw material flows. The LPCVD apparatus according to the present invention enables recovery of a larger amount of used raw material without reducing its exhaust efficiency.

This nonprovisional application claims the benefit of U.S. ProvisionalApplication No. 60/254,193, filed Dec. 11, 2000.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to an apparatus for carrying out a processfor forming a metal film or a metallic compound film with the use ofLPCVD method, also relates to a method for controlling the filmformation process.

2. Description of the Related Art

Since chemical vapor deposition method (hereinafter referred to as CVDmethod) can be used to produce a uniform film, and since CVD method hasan excellent capability called “step coverage” which can be effectivelyused to form a uniform film on an uneven surface, this method has becomeone of generally used thin film formation techniques for forming thinfilm electrodes in a semiconductor device.

A thin film formation process involved in the CVD method requires thatafter a raw material has been vaporized and moved to the surface of asubstrate, the particles of the raw material are caused to react withone another on the substrate so as to be accumulated thereon.Particularly, in recent years, there has been in use a low pressure CVDmethod (hereinafter referred to as LPCVD method) which takes intoaccount the manufacturing of a thin film at a lower temperature but witha higher efficiency, and enables the above reaction to be conducted inan atmosphere having a reduced pressure. Further, in order to reduce thereaction temperature, as a metallic compound for use as a raw material,there has been in use an organometallic compound.

Here, an apparatus for manufacturing a thin film using the LPCVD method,is comprised of a container for accommodating an organometallic compoundas a raw material, a heating device for heating the container tovaporize the organometallic compound so as to obtain a raw material gas,and a reactor accommodating a substrate for precipitating a thin film,with all of them being communicated in a predetermined manner by virtueof various pipes. In addition, the LPCVD apparatus is equipped with anexhaust pump which is used to maintain a low pressure atmosphere withinthe reactor. Furthermore, in order for the exhaust pump to be kept at ahigh exhaust efficiency and in order to prevent any possible failureduring the operation of the exhaust pump, there has been in use a trap(cold trap) for removing in advance some condensed components from theexhaust gas.

However, the inventors of the present invention have recently developeda recycle technique in order to solve an existing problem, i.e., thecost for manufacturing a thin film is relatively high because of a lowefficiency in using the raw material compound in the CVD method.According to the improved recycle technique of the invention, the rawmaterial used in the above reaction is recovered, and an un-reactedmetallic compound is refined from the used and recovered raw material sothat the un-reacted metallic compound can be processed into a reusableform. Moreover, the inventors of the present invention have found iteffective to use a trap to recover the used raw material, because theuse of the trap makes it easy to effect the recovery of the used rawmaterial. Namely, a raw material compound (organometallic compound) usedin the CVD method usually has a relatively low boiling point and arelatively low vapor pressure, so that such a raw material compound iseasy to undergo a phase change at a relatively low temperature and thusit may be easily condensed from its gaseous phase into its liquid phase.Accordingly, if taking into account the original purpose of using atrap, a trap may be considered to be an excellent device for recoveringthe used raw material. Besides, since an LPCVD apparatus is usuallyequipped with a trap, it is not necessary to provide another trap toeffect the recovery of the used raw material.

On the other hand, when using the recycle technique described in theabove, it is usually desired to recover as much as possible the used rawmaterial in order to be able to recycle as much as possible the metalliccompound. Accordingly, in order to increase the recovery amount whenrecovering the used raw material, it is usual to consider increasing thecapacity of a trap.

However, an increase in the capacity of a trap will undesirably cause anincrease in the size of a CVD apparatus, thus causing an increase in theequipment cost. Moreover, an increase in the capacity of a trap can alsoreduce an exhaust efficiency of an exhaust pump, thus undesirablyreducing the functions originally provided by the trap.

Accordingly, the present invention is to provide an improved LPCVDapparatus capable of recovering a larger amount of used raw material,without reducing its exhaust efficiency.

SUMMARY OF THE INVENTION

The inventors of the present invention, as a result of investigation forsolving the above-described problems, have thought that in order toefficiently recover the used raw material, it is important to increase acooling efficiency when cooling a used raw material in the trap.Further, as a method for putting such an idea into practical use, theinventors think it is appropriate to fill the gas flowing passage of thetrap with an amount of solid fillers. In a commonly used conventionaltrap, the internal surfaces thereof can serve as effective coolingsurfaces. However, since in such a conventional trap there is only asmall contact area allowing a contact between a used material and thecooling surfaces, a sufficient cooling of the used raw material isalmost impossible. In contrast to this, using an amount of fillers tofill the gas flowing passage of the trap can increase the contact areaallowing the desired contact between a used material and the coolingsurfaces, thereby ensuring an increased cooling efficiency in the trap.

On the other hand, for use as the fillers it is usual to employpellet-like materials such as Raschig rings. As this time, in order toincrease the cooling efficiency for cooling the used raw material, anamount of fillers are required to be loaded into the trap with a highdensity. However, if an amount of fillers are loaded into the trap witha high density, a pressure loss across the trap will be large, and thiswill make it difficult for an exhaust pump to carry out an exhaustoperation. As a result, a pressure dropping speed in the reactor will bereduced, bringing about an undesired influence to the reaction in whicha thin film is formed. For this reason, the commonly used pellet-likefillers are considered to be not appropriate for being loaded into thetrap.

For the above reason, the inventors of the present invention think thatit is preferable to use some cylindrical fillers having a honeycombstructure, which can server as the fillers capable of efficientlycooling the used raw material, without bringing about any unfavourableinfluence to the exhaust efficiency of the exhaust pump.

Namely, the present invention relates to an LPCVD apparatus whichcomprises a container for accommodating an organometallic compoundserving as a raw material, a heating device for heating the container tovaporize the organometallic compound so as to obtain a raw material gas,and a reactor containing a substrate for precipitating a thin film, anexhaust pump for maintaining a low pressure atmosphere within thereactor, a cooling trap provided on the upstream side of the exhaustpump for cooling used raw material gas supplied from the reactor. Inparticular, the LPCVD apparatus is characterized in that a raw materialflowing passage within the trap is filled with an amount of cylindricalfillers having a honeycomb structure.

The fillers having a honeycomb structure which can be suitably used inthe present invention are so formed that the cross section of eachfiller has a plurality of holes formed therethrough, with these holesextending in the longitudinal direction (along which the used rawmaterial gas flows) of each cylindrical filler. Therefore, if an amountof fillers to be used are those having such a honeycomb structure, it ispossible to prevent an undesired increase in the pressure loss of theraw material gas passing therethrough. Further, since the fillers of thehoneycomb structure have a great number of holes surrounded by partitionwalls, it is allowed for each piece of filler to have a large surfacearea. Accordingly, the fillers of the honeycomb structure can beconsidered to be excellent as having an excellent cooling efficiency forcooling the used raw material gas.

As may be understood from the above description, the LPCVD apparatus ofthe present invention is capable of reducing the internal pressure inthe reactor with an exhaust efficiency which is the same as that of theconventional LPCVD apparatus, thereby exhibiting the same efficiency formanufacturing the thin film as in the conventional apparatus. Meanwhile,using the LPCVD apparatus of the present invention makes it possible torecover the used raw material with an efficiency higher than that in theprior art. Therefore, it has become possible to recycle a large amountof raw material metallic compound, thus making it possible to reduce thecost for manufacturing the thin film.

As to a material forming the fillers of the honeycomb structure forfilling the trap, it is preferable to employ a metal. This is because ametal usually has a high thermal conductivity, so that using a metal toform the fillers makes it possible to efficiently cool the used rawmaterial. Further, as to the size of the fillers, it is preferable thatthe length thereof is in a range of 0.01 to 1.0 m. If the length isshorter than 0.01 m, it will be difficult to effect a sufficient coolingof the used raw material. On the other hand, if the length is longerthan 1.0 m, not only an undesired influence will be brought about to thecooling efficiency, but also the trap has to be made large in its sizeand this will cause a decrease in the exhaust efficiency. Besides, as tothe size of the holes formed in each filler of the honeycomb structure,it is preferable that the diameter of the holes is in a range of 0.5 to10 mm. If the hole diameter is smaller than 0.5 mm, a pressure losscaused due to the fillers will be large, and the holes will get chokedby the used and condensed raw material. On the other hand, if the holediameter is larger than 10 mm, it will be difficult to cool the used rawmaterial to a sufficient extent.

During the thin film formation process using the LPCVD apparatus of thepresent invention, an internal pressure in the trap is preferred to bekept at a value which is equal to or slightly lower than an internalpressure in the reactor. Since most of the metallic compounds (organiccompounds) used in the CVD method have a relatively low vapor pressure,they are not easy to be condensed under a low pressure and at a lowtemperature. As a result, it is necessary for the internal pressure inthe trap to be kept at a relatively high value. In practice, theinternal pressure in the trap is preferred to be kept in a range havinga lower limit which is equal to or higher than 0.1 Pa, and an upperlimit which is equal to or higher than the internal pressure in thereactor.

Furthermore, if taken into account the control of the internal pressurein the trap, the LPCVD apparatus of the present invention is preferredto be equipped with a trap pressure regulating valve which is locatedbetween the trap and the exhaust pump for adjusting the internalpressure in the trap. Further, it is preferable to provide a by-passpipe between the reactor and the exhaust pump. In this way, the internalpressure in the trap is allowed to be maintained at a value which isequal to or slightly lower than the internal pressure in the reactor.Namely, during the process of manufacturing the thin film using theLPCVD apparatus of the present invention, the trap pressure regulatingvalve is operated by being opened and closed properly, so that an amountof an exhaust gas discharged from the trap can be regulated and theinternal pressure in the trap may be controlled within theabove-described desired range. On the other hand, the discharge of theexhaust gas from the reactor can be continued exactly by virtue of theby-pass pipe provided between the reactor and the exhaust pump (even ifthe trap pressure regulating valve is in its closed position), therebymaking it possible to constantly maintain the internal pressure in thetrap. For example, if the internal pressure in the trap becomes lowerthan a required value, the pressure regulating valve may be closed so asto shut off the trap. At this time, since the discharge of the exhaustgas from the reactor can be continued by way of the by-pass pipe, it isallowed to continue the thin film formation process without increasingthe internal pressure in the reactor.

Further, in order to replace a treated substrate with an untreated one,an actual process for manufacturing a thin film is usually carried outin a manner of one batch after another, in which the operation of theCVD apparatus is stopped temporarily and the reactor is at first openedand then closed for replacing a treated substrate with an untreated one.At this time, whenever the reactor is at first opened and then closed,it is necessary that the reactor be at first subjected to a pressurereduction and then to a pressure increase. In fact, a conventional LPCVDapparatus is so formed that when the internal pressure in the reactor isincreased, the internal pressure in the trap will also be increased atthe same time. In this way, if it is desired to again reduce theinternal pressure in the reactor, the internal pressure in the trap isalso needed to be reduced. Different from the conventional LPCVDapparatus, the LPCVD apparatus of the present invention is equipped witha pressure regulating valve on the downstream side of the trap, as wellas a by-pass pipe connected between the reactor and the exhaust pump.Thus, by properly operating the pressure regulating valve and theby-pass pipe, it is possible to carry out the thin film manufacturingprocess with a high efficiency. Namely, since the pressure regulatingvalve is made operative only during the thin film manufacturing process,and since the pressure regulating valve is closed when a treatedsubstrate is being replaced by an untreated one (so that the trap isshut off and thus a reduced pressure can be maintained), it is notnecessary to discharge an exhaust gas having an amount corresponding tothe capacity of the trap (during this time the internal pressure in thereactor is being reduced), thereby making it sure to reduce the internalpressure in the reactor with a high efficiency.

However, in the LPCVD apparatus of the present invention, besides thetrap pressure regulating valve and the by-pass pipe, there is alsoprovided a back-flow valve on the upstream side of the trap in order toprevent a back flow of a fluid from the trap. In addition, it ispreferable to provide the by-pass shut-off valves for shutting off anundesired by-pass flow so as to prevent a used raw material gas (flowingfrom the reactor during the thin film manufacturing process) fromdirectly flowing into the exhaust pump.

The thin film manufacturing process using the LPCVD apparatus of thepresent invention will be described with reference to FIG. 1. The LPCVDapparatus shown in FIG. 1 can be considered to be the most preferableembodiment of the present invention, which comprises a trap pressureregulating valve, a by-pass pipe, a back-flow valve, two by-passshut-off valves, and a reaction pressure regulating valve for regulatingthe internal pressure in the reactor.

As shown in FIG. 1, at the start of the operation of the apparatus (asan initial step involved in the thin film manufacturing process), allthe valves are opened so as to reduce the internal pressures in both thereactor and the trap, as shown in FIG. 1(a).

Then, after the internal pressure in the reactor has reached apredetermined value suitable for manufacturing a thin film, the rawmaterial gas is introduced and the thin film formation process isstarted. At this time, the trap pressure regulating valve is opened orclosed properly, while at the same time the internal pressure in thetrap is monitorred, as shown in FIG. 1(b). Further, the by-pass shut-offvalves are closed so as to prevent the used raw material gas (flowingfrom the reactor) from directly flowing into the exhaust pump.

On the other hand, after the thin film manufacturing process has beenfinished and a treated substrate within the reactor is to be replaced bya new one, both the trap pressure regulating valve and the back-flowvalve are closed so as to shut off the trap, followed by increasing theinternal pressure in the reactor. After the new substrate has beenattached into the reactor, the internal pressure in the reactor isreduced. At this time, the trap pressure regulating valve is closed,while the by-pass shut-off valves are opened, so that the internalpressure in the reactor is reduced by way of the by-pass shut-offvalves, as shown in FIG. 1(c).

However, various valves after the substrate replacement are repeatedlyoperated through the operations shown in FIGS. 1(b) and 1(c).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a thin film manufacturing process using theLPCVD apparatus formed according to the present invention.

FIG. 2 is a schematic view showing the LPCVD apparatus used in thepresent embodiment.

FIG. 3 is a schematic view showing a trap for use in the LPCVD apparatusemployed in the present embodiment.

FIG. 4 is a schematic view showing a conventional LPCVD apparatus usedin a comparative example.

EMBODIMENT FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 2 is a schematic view showing an LPCVD apparatus used in thepresent embodiment. The LPCVD apparatus 1 shown in FIG. 2 comprises athermostat 2 serving as a container for enclosing an amount oforganometallic compound, a reactor 3, a trap 4 for cooling andcondensing a raw material gas used in the reaction, and an exhaust pump5 for reducing an internal pressure in the reactor 3 and an internalpressure in the trap 4. The reactor 3 contains a substrate 6 and aheater 7 for heating the substrate 6. Further, in the presentembodiment, a trap pressure regulating valve 8 for regulating aninternal pressure in the trap 4 is disposed between the trap 4 and theexhaust pump 5. Moreover, a reactor pressure regulating valve 9 forregulating an internal pressure in the reactor 3 is provided on thedownstream side of the reactor 3. A back-flow valve 10 is provided onthe upstream side of the trap 4. In addition, between the downstreamside of the reactor pressure regulating valve 9 and the upstream side ofthe exhaust pump 5, there is connected a by-pass pipe 12 having twoshutoff valves 11 disposed one on each end thereof.

In the LPCVD apparatus 1, an amount of organometallic compound 13serving as a raw material is heated in the thermostat 2 so as to bevaporized and thus become a desired raw material gas. The raw materialgas is then mixed with an oxygen gas 14 serving as a carrier gas. Themixed gas is then transported to the surface of the substrate 6 disposedwithin the reactor 3. Afterwards, the substrate 6 is heated by theheater 7 so as to effect a CVD reaction on the surface of the substrate6.

FIG. 3 is used to illustrate in detail the trap 4. As shown in thefigure, the trap 4 is so formed that a used raw material gas 22 suppliedfrom the reactor 3 is introduced into the trap 4 through an introductionopening 21 formed on one side of the trap, then discharged upwardlythrough a center pipe 23. Further, the passage for passing the used rawmaterial gas is filled with an amount of fillers 24 having a honeycombstructure. Here, the material for forming the honeycomb fillers 24 is astainless steel. In addition, a cooling water 25 is caused to flow alongthe wall surfaces of the trap 4.

A ruthenium thin film was manufactured with the use of the LPCVDapparatus 1, under conditions indicated below.

Raw material: bis(ethylcyclopentadienyl) ruthenium 200 g

Raw material supply speed: 0.05 g/min

Substrate: Ti-coated Si substrate

Substrate temperature: 300° C.

Pressure within reaction chamber: 133.3 Pa (1.0 torr)

Carrier gas flow rate: 50 mL/min

Reaction gas flow rate: 200 mL/min

Here, a coolant (water) is caused to flow over the cooling surfaces ofthe cold trap 4 so as to cool the gas passing therethrough to atemperature of 10° C. Further, the pressure within the cold trap iscontinuously monitored and controlled to a value of 133.3 Pa (1.0 torrwhich is the same pressure as that within the reactor 3) by the pressureregulating valve 8.

The thin film formation process was continued until the raw material wasall used up. Upon collecting the recovered components caught in the trap4, it was found by calculation that the recovered components was 154 g,indicating a recovery rate of 77%.

However, when the recovered components were subjected to distillation ata temperature of 105° C. and a pressure of 46.7 Pa (0.35 torr), 142 g offractions were recovered, indicating a recovery rate of 71% (withrespect to an amount of the original raw material). After the fractionswere analyzed in a gas chromatography, it was found that the recoveredsubstance was bis(ethylcyclopentadienyl) ruthenium having a purity of99.56%.

COMPARATIVE EXAMPLE

FIG. 4 is used to illustrate a conventional LPCVD apparatus 30 used in acomparative example. Although the LPCVD apparatus 30 has the same basicstructure as that of the LPCVD apparatus 1 used in the above embodimentof the present invention, the structures of their traps are differentfrom each other. This is because the LPCVD apparatus 30 employs only acommonly used trap 31. Moreover, the LPCVD apparatus 30 does not containthe trap pressure regulating valve 8 and the by-pass pipe 12 (includingthe by-pass shut-off valves 11).

Thereafter, the LPCVD apparatus involved in the comparative example wasused to manufacture a thin film of ruthenium under the same condition asin the above embodiment of the present invention.

The internal pressure in the trap in the comparative example was 33×10⁻²Pa, the used raw material collected in the trap was 10 g, indicating arecovery rate of 5%. Thus, it is understood that the recovery rate inthe comparative example is lower than that in the above embodiment ofthe present invention, only because of the following two reasons. Thefirst reason is that the cooling efficiency within the trap in the aboveembodiment of the present invention is completely different from that inthe comparative example. The second reason may be that the internalpressure in the trap in the comparative example is too low, and as aresult, the raw material gas fails to be sufficiently condensed at acooling temperature of 10° C.

What is claimed is:
 1. An LPCVD apparatus comprising: a container foraccommodating an organometallic compound, said compound serving as a rawmaterial; a heating means for heating the container and vaporizing theorganometallic compound to obtain a raw material gas; a reactor foraccommodating a substrate on which a thin film is precipitated; anexhaust pump for maintaining a low pressure atmosphere within thereactor; and a trap provided on the upstream of the exhaust pump andcooling used raw material gas supplied from the reactor, wherein saidtrap is provided with metal honeycomb-structure cylindrical fillers in aflowing passage through which the used raw material flows.
 2. The LPCVDapparatus according to claim 1, wherein the length of thehoneycomb-structure cylindrical fillers is in a range of 0.01 to 1.0 min a direction along which the used raw material flows.
 3. Th LPCVDapparatus according to claim 1, wherein the honeycomb-structurecylindrical fillers have holes with a maximum diameter of 0.5 to
 10. 4.The LPCVD apparatus according to claim 1, wherein said apparatus isprovided with a trap-pressure-regulating valve for adjusting theinternal pressure in the trap, and the exhaust pump.
 5. The LPCVDapparatus according to claim 1, wherein said apparatus is provided witha back-flow valve for preventing a back flow of the used raw material inthe trap, said back-flow valve being located between the reactor and thetrap.
 6. The LPCVD apparatus according to claim 1, wherein saidapparatus is connected with a first and a second pipes and provided witha by-pass pipe which bypasses the trap, said first pipe connecting thereactor and the trap and said second pipe connecting the trap and thepump.
 7. The LPCVD apparatus according to claim 1, wherein said by-passpipe is provided at the both ends thereof with a back-flow valve.